What makes Radiohead so exhilarating on a big stage is their ability to persuade huge audiences to travel with them far outside the standard arena-rock comfort zone. To see a festival crowd twitching to the bone-dry electro of Idioteque is to see horizons broadening. That frisson is, inevitably, impossible to match in a theatre where 300 die-hard fans occupy plush red seats, radiating such reverence that Thom Yorke prompts roaring hilarity simply by saying "Thank you very much" in a funny voice.

But the modest dimensions of this Radio 2 session fit the intimacy of last year's In Rainbows album (which forms the bulk of the set) and allow the spectator to observe exactly how this unorthodox group operates. Most big bands' idea of sonic derring-do extends to hiring a keyboard player, but Radiohead are constantly rearranging themselves into curious new shapes, sufficiently egoless to do only what the song requires.

For much of Reckoner, three members do no more than shake percussion instruments. Jonny Greenwood treats his guitar like his synthesisers: just another machine for making strange and beautiful noises. If they start out as a conventional rock band with the writhing, three-guitar attack of Bodysnatchers, they end up resembling the BBC Radiophonic Workshop, hunkered down over electronic devices during Everything in Its Right Place, from 2000's Kid A album. Yorke feeds his voice into a sampler, then walks off, leaving it to squiggle around inside the machine like a fly in a bottle. The frontman effectively disassembles himself.

That is why they are admired, but they are loved because they tap a deep well of emotion. In Rainbows is their most unashamedly gorgeous collection of songs to date - House of Cards is practically a soul record - and Yorke's voice is increasingly preoccupied with sound rather than meaning. His words often melt into a moan, or vaporise, snake upwards and merge with the music, which happens on the exquisite Nude. There is a lovely moment at the start of Weird Fishes/Arpeggi when Yorke and guitarist Ed O'Brien exchange smiles, which by Radiohead's reserved standards is the equivalent of standing back to back, soloing wildly.

After the recording finishes, Yorke and Greenwood reappear for an encore. As they ready themselves, some wag who obviously did not get the memo re: reverence shouts, "Come on!" "You in a hurry to go?" Yorke teases. And, after a tender, acoustic reading of Faust Arp, he does.